Drug addiction is the habit of drug use, generally under little self-control. The addictive nature of some drugs causes people to acquire a perceived dependence on them.

This dependence appears to be caused by the drugs' action on the central nervous system, where they appear to stimulate the neurotransmitter pathways of the brain by a variety of actions, such as simulating the effects of natural neurotransmitters such as serotonin or dopamine.

When drug stimulation ceases, a variety of withdrawal symptoms may be felt, commonly including mental depression.

The desire for continued drug stimulation, and countering these withdrawal effects, is thought to be what drives drug-users to pursue more stimulants, causing a increasingly destructive cycle of psychological dependency on a stimulant or stimulants.

In addition, a large number of other substances are of concern as illegal drugs, the most famous of which are heroin, cocaine, cannabis and MDMA.

In jurisdictions where addictive drugs are illegal, they are generally supplied by criminal drug dealers, often in the form of organized crime.
The addict's need to support the high cost of illegal addictive drugs is one of the major causes of crime.

The War on Drugs is a U.S. attempt to reduce the harm caused by drug addiction and crime. Critics of the War on Drugs argue that like Prohibition, the 1920s attempt to control alcohol, the War on Drugs may actually be counter-productive. Instead, they call for the total or partial legalization of currently illegal addictive drugs.